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Cassava Botany and Physiology

Cassava Botany and Physiology

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Chapter 5 Cassava and Physiology

Alfredo Augusto Cunha Alves Embrapa Cassava and , Caixa Postal 007, 44.380–000, Cruz das Almas, Bahia, Brazil

Introduction cuttings. The seedlings are genetically segre- gated into different types due to their Cassava is a perennial of the family reproduction by cross-. If propagated , cultivated mainly for its starchy by cuttings under favourable conditions, sprout- . It is one of the most important food staples ingandadventitiousrootingoccurafter1week. in the tropics, where it is the fourth most impor- tant source of energy. On a worldwide basis it is ranked as the sixth most important source of Morphological and Agronomic calories in the human diet (FAO, 1999). Given Characteristics the crop’s tolerance to poor soil and harsh clima- tic conditions, it is generally cultivated by small Cassava, which is a shrub reaching 1–4 m farmers as a subsistence crop in a diverse range height, is commonly known as tapioca, manioc, of agricultural and food systems. Although mandioca and yuca in different parts of the cassava is a perennial crop, the storage roots can world. Belonging to the dicotyledon family be harvested from 6 to 24 months after planting Euphorbiaceae, the Manihot genus is reported to (MAP), depending on and the growing have about 100 species, among which the only conditions (El-Sharkawy, 1993). In the humid commercially cultivated one is Manihot esculenta lowland tropics the roots can be harvested after Crantz. There are two distinct types: erect, 6–7 months. In regions with prolonged periods with or without branching at the top, or of drought or cold, the farmers usually harvest spreading types. after 18–24 months (Cock, 1984). Moreover, The morphological characteristics of the roots can be left in the ground without cassava are highly variable, which indicate a harvesting for a long period of time, making it high degree of interspecific hybridization. There a very useful crop as a security against famine are many cassava in several germplasm (Cardoso and Souza, 1999). banks held at both international and national Cassava can be propagated from either stem research institutions. The largest germplasm cuttings or sexual , but the former is the bank is located at Centro Internacional de commonest practice. Propagation from true seed Agricultura Tropical (CIAT), Colombia, with occurs under natural conditions and is widely approximately 4700 accessions (Bonierbale used in breeding programmes. from true et al., 1997), followed by EMBRAPA’s collection seed take longer to become established, and they in Cruz das Almas, Bahia, with around 1700 are smaller and less vigorous than plants from accessions (Fukuda et al., 1997), representing

©CAB International 2002. Cassava: Biology, Production and Utilization (eds R.J. Hillocks, J.M. Thresh and A.C. Bellotti) 67

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the germplasm of the following Brazilian eco- under the soil. These roots develop to make systems: lowland and highland semiarid tropics, a fibrous system. Only a few fibrous roots lowland humid subtropics, lowland subhumid (between three and ten) start to bulk and become tropics, lowland humid tropics, and lowland hot storage roots. Most of the other fibrous roots savannah. The cassava genotypes are usually remain thin and continue to function in characterized on the basis of morphological and and nutrient absorption. Once a fibrous root agronomic descriptors. Recently, the Interna- becomes a storage root, its ability to absorb water tional Plant Genetic Resources Institute (IPGRI) and nutrients decrease considerably. The storage descriptors (Gulick et al., 1983) were revised, and roots result from of the fibrous a new version was elaborated, in which 75 roots; thus the soil is penetrated by thin roots, descriptors were defined, 54 being morpho- and their enlargement begins only after that logical and 21 agronomic (Fukuda et al., 1997). penetration has occurred. Morphological descriptors (for example, lobe Anatomically, the cassava root is not a shape, root pulp colour, stem external colour) tuberous root, but a true root, which cannot have a higher heritability than agronomic (such be used for vegetative propagation. The mature as root length, number of roots per plant and root cassava storage root has three distinct tissues: yield). Among morphological descriptors, the bark (periderm), peel (or cortex) and paren- following were defined as the minimum or basic chyma. The parenchyma, which is the edible descriptors that should be considered for identify- portion of the fresh root, comprises approxi- ing a cultivar: (i) apical colour; (ii) apical leaf mately 85% of total weight, consisting of xylem pubescence; (iii) central lobe shape; (iv) vessels radially distributed in a matrix of - colour; (v) stem cortex colour; (vi) stem external containing cells (Wheatley and Chuzel, 1993). colour; (vii) phyllotaxis length; (viii) root ped- The peel layer, which is comprised of uncule presence; (ix) root external colour; (x) sclerenchyma, cortical parenchyma and phloem, root cortex colour; (xi) root pulp colour; (xii) root constitutes 11–20% of root weight (Barrios texture; and (xiii) flowering. and Bressani, 1967). The periderm (3% of total Given the large number of cassava genotypes weight) is a thin layer made of a few cells thick cultivated commercially and the large diversity and, as growth progresses, the outermost por- of ecosystems in which cassava is grown, it is tions usually slough off. Root size and shape difficult to make a precise description of the mor- depend on cultivar and environmental condi- phological descriptors as there is a genotype- tions; variability in size within a cultivar is by-environmental conditions interaction. Thus, greater than that found in other root crops in addition to morphological characterization, (Wheatley and Chuzel, 1993). Table 5.1 lists molecular characterization, based mainly on morphological and agronomic characteristics of DNA molecular markers, has been very useful in the root and its variability in cassava. order to evaluate the germplasm genetic diver- sity(Beechingetal.,1993;Fregeneetal.,1994). Stems

Roots The mature stem is woody, cylindrical and formed by alternating nodes and internodes. On Roots are the main storage organ in cassava. the nodes of the oldest parts of the stem, there In plants propagated from true a typical are protuberances, which are the scars left by primary tap root system is developed, similar the plant’s first . A plant grown from stem to dicot species. The radicle of the germinating cuttings can produce as many primary stems as seed grows vertically downward and develops there are viable buds on the cutting. In some into a , from which adventitious roots cultivars with strong apical dominance, only originate. Later, the taproot and some adventi- one stem develops. tious roots become storage roots. The cassava plant has sympodial branch- In plants grown from stem cuttings the roots ing. The main stem(s) divide di-, tri- or tetra- are adventitious and they arise from the basal- chotomously, producing secondary branches cut surface of the stake and occasionally from the that produce other successive branchings. These

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Table 5.1. Some morphological and agronomic characteristics of roots and their variability in cassava.

Root characteristic Variability Reference

Morphological External colour White or cream; yellow; light Fukuda and Guevara (1998) brown; dark brown Cortex colour White or cream; yellow; pink; purple Fukuda and Guevara (1998) Pulp (parenchyma) colour White; cream; yellow; pink Fukuda and Guevara (1998) Epidermis texture Smooth; rugose Fukuda and Guevara (1998) Peduncule Sessile; pedunculate; both Fukuda and Guevara (1998) Constriction None or little; medium; many Fukuda and Guevara (1998) Shape Conical; conical–cylindrical; Fukuda and Guevara (1998) cylindrical; irregular Agronomic No. storage roots/plant 3–14 Dimyati (1994); Wheatley and Chuzel (1993); Ramanujam and Indira (1983); Pinho et al. (1995) Weight of storage roots/plant 0.5–3.4 kg FW Dimyati (1994); Wheatley and Chuzel (1993); Ramanujam and Indira (1983) Weight of one storage root 0.17–2.35 kg Barrios and Bressani (1967); Ramanujam and Indira (1983) Length of storage root 15–100 cm Wheatley and Chuzel (1993); Barrios and Bressani (1967); Pinho et al. (1995) Diameter of storage root 3–15 cm Wheatley and Chuzel (1993); Barrios and Bressani (1967); Pinho et al. (1995) Diameter of fibrous root 0.36–0.67 mm Connor et al. (1981) Depth of fibrous root Up to 260 cm Connor et al. (1981) Amylose in root starch 13–21% FW O’Hair (1990) Protein in whole root 1.76–2.68% FW Barrios and Bressani (1967) Protein in pulp (parenchyma) 1.51–2.67% FW Barrios and Bressani (1967) 1.0–6.0% DW Wheatley and Chuzel (1993) Protein in peel 2.79–6.61% FW Barrios and Bressani (1967) 7.0–14.0% DW Wheatley and Chuzel (1993) DM in whole fresh root 23–43% Barrios and Bressani (1967); Ghosh et al. (1988); Ramanujam and Indira (1983); O’Hair (1989) DM in peel 15–34% Wheatley and Chuzel (1993); Barrios and Bressani (1967); O’Hair (1989) DM in pulp 23–44% Wheatley and Chuzel (1993); Barrios and Bressani (1967); O’Hair (1989) in whole root 85–91% DW Barrios and Bressani (1967) Carbohydrates in peel 60–83% DW Barrios and Bressani (1967) Carbohydrates in pulp 88–93% DW Barrios and Bressani (1967) (parenchyma) Starch in whole root 20–36% FW Wholey and Booth (1979); O’Hair (1989); Ternes et al. (1978) 77% DW Ghosh et al. (1988) Starch in peel 14–25% FW O’Hair (1989) 44–59% DW Wheatley and Chuzel (1993) Continued

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Table 5.1. Continued.

Root characteristic Variability Reference

Starch in pulp (parenchyma) 26–40% FW O’Hair (1989) 70–91% DW Wheatley and Chuzel (1993) Peel in whole root 11–20% FW Barrios and Bressani (1967) Crude fibre in whole root 3.8–7.3% DW Barrios and Bressani (1967) Crude fibre in peel 9.2–21.2% DW Barrios and Bressani (1967) 5.0–15.0% DW Wheatley and Chuzel (1993) Crude fibre in pulp 2.9–5.2% DW Barrios and Bressani (1967) (parenchyma) 3.0–5.0% DW Wheatley and Chuzel (1993) Total 1.3–5.3% DW Wheatley and Chuzel (1993)

DM, dry matter; FW, fresh weight; DW, dry weight.

branchings, which are induced by flowering, 0.5–1.0 cm long), which remain attached to the have been called ‘reproductive branchings’. stem when the leaf is completely developed Stem morphological and agronomic char- (CIAT, 1984). The petiole length of a fully opened acteristics are very important to characterizing leaf normally varies from 5to 30 cm, but may a cultivar (Table 5.2). The variation of these reach up to 40 cm. characteristics depends on cultivar, cultural The upper leaf surface is covered with a practice and climatic conditions. shiny, waxy epidermis. Most stomata are located on the lower (abaxial) surface of the leaves; only a few can be found along the main vein on Leaves the upper (adaxial) surface (Cerqueira, 1989). Of 1500 cultivars studied, only 2% had stomata Cassava leaves are simple, formed by the lamina on the adaxial surface (El-Sharkawy and Cock, and petiole. The leaf is lobed with palmated 1987a). The stomata on the upper surface are veins. There is generally an uneven number of also functional and bigger than those on the lobes, ranging from three to nine (occasionally undersurface. Both are morphologically para- 11). Only a few cultivars are characterized by cytic, with two small guard cells surrounded having three-lobed mature vegetative leaves, by two subsidiary cells (Cerqueira, 1989). The which may represent the primitive ancestral number of stomata per leaf area range from form (Rogers and Fleming, 1973). Leaves near 278 to 700 mm−2, and all stomatal pores can the are generally reduced in size occupy from 1.4 to 3.1% of the total leaf area and lobe number (most frequently three-lobed), (Table 5.3). but the one closest to the base of the inflores- cence is frequently simple and unlobed. Leaves are alternate and have a phyllotaxy of 2/5, indicating that from any leaf (leaf 1) there are two revolutions around the stem to reach the Cassava is a monoecious species producing both sixth (leaf 6) in the same orthostichy as leaf 1. In male (pistillate) and female (staminate) flowers these two revolutions there are five successive on the same plant. The inflorescence is generally intermediate leaves (not counting leaf 1). formed at the insertion point of the reproductive The main leaf morphological and agro- branchings; occasionally can be nomic characteristics and their variation are found in the leaf axils on the upper part of the given in Table 5.3. Many of them (mainly the plant. The female flowers, located on the lower morphological ones) are used to characterize part of the inflorescence, are fewer in number cultivars and may vary with environmental than male flowers, which are numerous on the conditions and plant age. upper part of the inflorescence. On the same Mature leaves are glabrous and each leaf inflorescence, the female flowers open 1–2 is surrounded by two stipules (approximately weeks before the male flowers (protogyny).

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Table 5.2. Some morphological and agronomic characteristics of stems and their variability in cassava.

Stem characteristic Variability Reference

Morphological Cortex (collenchyma) colour Yellow; light green; dark green Fukuda and Guevara (1998) External colour Orange; green–yellow; gold; dark Fukuda and Guevara (1998) brown; silver; grey; dark brown Phyllotaxis length Short (< 8 cm); medium (8–15 cm); Fukuda and Guevara (1998) large (> 15 cm) Epidermis colour Cream; light brown; dark brown; Fukuda and Guevara (1998) orange Growth Straight; zigzag Fukuda and Guevara (1998) Apical stem colour Green; green-purple; purple Fukuda and Guevara (1998) Branching habit Erect; dichotomous; trichotomous; Fukuda and Guevara (1998) tetrachotomous Agronomic Diameter of mature stem 2–8 cm CIAT (1984); Ramanujam and Indira (1983) Plant height 1.20–3.70 m Ramanujam and Indira (1983); Ramanujam (1985); Veltkamp (1985a); Pinho et al. (1995) No. of nodes from planting– 22–96 Veltkamp (1985a) 1st branch level No. of days from planting– 49–134 Veltkamp (1985a) 1st branch level No. of apices/plant 2.8–27.5 Pinho et al. (1995)

Male and female flowers on different branches three undulated, fleshy lobes originates from the of the same plant can open at the same time. style. Normally, cassava is cross-pollinated by insects; thus it is a highly heterozygous plant. The flowers do not have a calyx or corolla, and seeds but an indefinite structure called or perigonium, made up of five yellow, reddish or The fruit is a trilocular , ovoid or globu- purple . The male is half the size of lar, 1–1.5 cm in diameter and with six straight, the female flower. The of the male flower prominent longitudinal ridges or aristae. Each is thin, straight and very short, while that of the locule contains a single carunculate seed. The female flower is thick, curved and long. Inside the fruit has a bicidal dehiscence, which is a combi- male flower, there is a basal disk divided into ten nation of septicidal and loculicidal dehiscences, lobes. Ten originate from between them. with openings along the parallel plane of the They are arranged in two circles and support the dissepiments and along the midveins of the anthers. The five external stamens are separated carpels, respectively. With this combination of and longer than the inner ones, which join dehiscences, the fruits open into six valves caus- together on the top to form a set of anthers. The ing an explosive dehiscence, ejecting the seeds is generally yellow or orange, varying some distance (Rogers, 1965). Fruit maturation from 122 to 148 µm in size, which is very large generally occurs 75–90 days after pollination compared to other flowering plants (Ghosh et al., (Ghosh et al., 1988). The seed is ovoid– 1988). The female flower also has a ten-lobed ellipsoidal, approximately 100 mm long, 6 mm basal disk, which is less lobulated than the male wide and 4 mm thick. The weight varies from 95 flower. The is tricarpellary with six ridges to 136 mg per seed (Ghosh et al., 1988). The and is mounted on the basal disk. The three smooth seed coat is dark brown, mottled with locules contain one each. A very small style grey. The seeds usually germinate soon after col- is located on top of the ovary, and a with lection, taking about 16 days for .

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Table 5.3. Some morphological and agronomic characteristics of leaves and their variability in cassava.

Leaf characteristic Variability Reference

Morphological Apical leaf colour Light green; dark green; green–purple; Fukuda and Guevara (1998) purple Apical pubescence Absent; present Fukuda and Guevara (1998) Shape of central lobe Ovoid; elliptic–lanceolate; obovate– Fukuda and Guevara (1998) lanceolate; oblanceolate; lanceolate; linear; pandurate; linear–pyramidal; linear–pandurate; linear–hostatilobada Petiole colour Green–yellow; green; green–red; Fukuda and Guevara (1998) red-green; red; purple Mature leaf colour Light green; dark green; green-purple; Fukuda and Guevara (1998) purple Protuberance of leaf scars No protuberance; protuberant Fukuda and Guevara (1998) No. of lobes 3; 5; 7; 9; 11 Fukuda and Guevara (1998) Agronomic Petiole length 5–30 cm Ghosh et al. (1988) 9–20 cm CIAT (1984) Total 2.18–2.86 mg g−1 leaf FW Ramanujam and Jos (1984) Central lobe length 4–20 cm CIAT (1984) Central lobe width 1–6 cm CIAT (1984) No. of stomata/leaf area 278–700 mm−2 Ghosh et al. (1988); Cerqueira in adaxial epidermis (1989); Splittstoesser and Tunya (1992); Connor and Palta (1981) Relative area of stomata 1.4–3.1% Cerqueira (1989); Pereira and pore (% from leaf area) Splittstoesser (1990) Stipule length 0.5–1.0 cm CIAT (1984) Leaf thickness 100–120 µm Pereira and Splittstoesser (1990) DM in mature leaf 25% Barrios and Bressani (1967) Fibre in mature leaf 4.58% DW Barrios and Bressani (1967) Ash in mature leaf 8.28% DW Barrios and Bressani (1967) Protein in mature leaf 7.1–8.9% FW Barrios and Bressani (1967) 28.8% DW Barrios and Bressani (1967) Soluble carbohydrates 11.36% FW Barrios and Bressani (1967) 44.84% DW Barrios and Bressani (1967)

DM, dry matter; FW, fresh weight; DW, dry weight. Growth and Development depend on several factors related to varietal dif- ferences, environmental conditions and cultural Plant developmental stages practices. The initial growth (at 15-day inter- vals) from emergence to 150 days is presented in As cassava is a perennial shrub it can grow Fig. 5.2. Growth at 60-day intervals during the indefinitely, alternating periods of vegetative first cycle (0–360 days after planting; DAP) is growth, storage of carbohydrates in the roots, shown in Fig. 5.3. The results in these two fig- and even periods of almost dormancy, brought ures are consistent with other authors (Howeler on by severe climatic conditions such as low and Cadavid, 1983; Ramanujam and Biradar, temperature and prolonged water deficit. There 1987; Távora et al., 1995; Peressin et al., 1998). is a positive correlation between the total The periods and main physiological events biomass and storage root biomass (Fig. 5.1; during the growth of a cassava plant under Ramanujam, 1990). During its growth, there favourable conditions in the field can be are distinct developmental phases. The occur- visualized in these figures and are summarized rence, duration and existence of each phase below:

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Botany and Physiology 73

Fig. 5.1. Relationship between dry weight (DW) of whole plant (x) and DW of storage roots (y) for individual plants of a field trial at the University of the West Indies: y = 0.56x−34; r 2 = 0.96; n = 112. (Source: Boerboom, 1978.)

Fig. 5.2. Partitioning of dry matter during the initial development of cassava cv. Cigana, Cruz das Almas, Bahia, Brazil. (Source: Porto, 1986.)

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Fig. 5.3. Growth of cassava plant during the first cycle (12 months). Average of two varieties. DAP, days after planting. Graph made from data of Lorenzi (1978).

Emergence of sprouting – 5–15 DAP storage roots represent 10–15% of total dry matter (DM; Fig. 5.2). • From 5–7 DAP the first adventitious roots arise from the basal cut surface of the stake and occasionally from the buds under the Development of stems and leaves (canopy soil. establishment) – 90–180 DAP • 10–12 DAP the first sprouting occurs, • Maximum growth rates of leaves and stems followed by small leaves which start to are achieved in this period, and the branch- emerge (Conceição, 1979). ing habit and plant architecture is defined • Emergence is achieved at 15 DAP. (Fig. 5.3). • From 120 to 150 DAP the leaves are able to Beginning of leaf development and formation intercept the most of the incident light on of root system – 15–90 DAP canopy (Veltkamp, 1985c). • The true leaves start to expand around 30 • Maximum canopy size and maximum DM DAP (Fig. 5.2) when the photosynthetic partition to leaves and stems are accom- process starts to contribute positively to plished (Howeler and Cadavid, 1983; plant growth. Ramanujam, 1985; Távora et al., 1995). • Until 30 DAP, and root growth • The storage root continues to bulk. depends on the reserves of the stem cutting. • The most active vegetative growth for • The fibrous roots start to grow, replacing cassava occurs in this period (Ramanujam, the first adventitious roots. These new roots 1985). start to penetrate in the soil, reaching 40–50 cm deep, and function in water and High translocation to roots – nutrient absorption (Conceição, 1979). 180–300 DAP • Few fibrous roots (between three and 14) will become storage roots, which can be • Photoassimilate partition from leaves to distinguished from fibrous roots from 60 to roots is accelerated, making the bulking 90 DAP (Cock et al., 1979). At 75 DAP the of storage roots faster (Fig. 5.3).

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• The highest rates of DM accumulation In cassava a positive correlation between the in storage roots occur within this period leaf area or leaf area duration and yield of stor- (Fig. 5.3; Boerboom, 1978; Távora et al., age roots has been reported, indicating that leaf 1995; Peressin et al., 1998). area is crucial in determining crop growth rate • Leaf senescence increases, hastening rate of and the storage bulking rate of cassava (Sinha leaf fall (Fig. 5.3). and Nair, 1971; Cock, 1976; Cock et al., 1979). • Stem becomes lignified (Conceição, 1979). For cassava the leaf area per plant depends on the number of active apices (branching Dormancy – 300–360 DAP pattern), the number of leaves formed/apex, leaf size and leaf life. Given that there are significant • Rate of leaf production is decreased. varietal variations and influence of environmen- • Almost all leaves fall and shoot vegetative tal conditions (Veltkamp, 1985a), it is important growth is finished. to characterize the development of cassava leaf • Only translocation of starch to root is kept, area and its components. Table 5.4 lists values and maximum DM partition to the roots is of some parameters related to leaf growth that attained. have been found in cassava. These parameters • This phase occurs mainly in regions with are discussed below. significant variation in temperature and After leaf emergence (folded, 1 cm long) rainfall. and under normal conditions, the cassava leaf • The plant completes its 12-month cycle, reaches its full size on days 10–12. Leaf life (from which can be followed by a new period of emergence to abscission) depends on cultivar, vegetative growth, DM accumulation in the shade level, water deficit and temperature (Cock roots and dormancy again. et al., 1979; Irikura et al., 1979). It ranges from 40 to 210 days (Table 5.4), but is commonly 60–120 days (Cock, 1984). Leaf area development There are marked differences in leaf size among the different cultivars, and the size varies The analyses of crop growth and yield are with the age of the plant. The leaves produced usually evaluated on the basis of two parame- from 3 to 4 MAP are those that become the larg- ters: leaf area index (LAI), i.e. leaf area per unit est; maximum total leaf area is reached from 4 to ground area, and net assimilatory rate (NAR), 5 MAP (Cock et al., 1979; Irikura et al., 1979). i.e. the rate of DM production per unit leaf area. Leaf size is influenced by changing the branching

Table 5.4. Parameters related to leaf growth during the first cycle (12 months) and some values found in cassava.

Leaf growth parameter Value Reference

Individual leaf area 50–600 cm2 Ramanujam (1982); Splittstoesser and Tunya (1992); Veltkamp (1985a) Expansion period (from emergence to 12 days Conceição (1979) full size) Leaf longevity (from fully expanded to 36–100 days Ramanujam and Indira (1983); abscission) Conceição (1979) Leaf life (from emergence to abscission) 40–210 days Splittstoesser and Tunya (1992); Irikura et al. (1979); Ramanujam (1985); Veltkamp (1985a) No. leaves retained/plant 44–146 Ramanujam and Indira (1983) Leaf production rate/shoot 4–22 week−1 Ramanujam and Indira (1983) Leaf shedding rate/plant 10–24 leaves week−1 Ramanujam and Indira (1983) Total leaf area 1.24–3.38 m2 Ramanujam and Indira (1983) Cumulative no. of leaves/apex 117–162 Veltkamp (1985a); Cock et al. (1979)

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pattern. Larger leaves are produced when the number of active apices is reduced (Tan and Cock, 1979). The rate of leaf formation decreases with plant age and is lower at low temperatures (Irikura et al., 1979). Differences in mean LAI are closely related to the rate of root bulking. The optimal LAI for storage root bulking rate is 3–3.5 (Cock et al., 1979) and exists over a wide range of tempera- ture (Irikura et al., 1979). Initial leaf area devel- opment is slow, taking 60–80 DAP before an LAI of 1.0 is reached. From 120 to 150 DAP the light interception by the canopy is around 90% with an LAI of 3 (Veltkamp, 1985a). In order to obtain high storage root yields, the crop should reach an LAI of 3–3.5 as quickly as possible and maintain Fig. 5.4. Temporal development of cell division that LAI for as long as possible (Cock et al., 1979; and cell expansion processes during cassava leaf development in adaxial epidermal cells. (Source: Veltkamp, 1985a). Substantial leaf abscission Alves, 1998.) began at LAI values of 5.0–6.0 (Keating et al., 1982a). accumulation in the leaves decreases during the crop cycle. Until 60–75 DAP, cassava accumu- Temporal development of cell division lates DM more in leaves than in stems and and cell expansion storage roots, not including the stem cutting (Fig. 5.2). Then the storage roots increase Leaf area development is largely dependent on rapidly, reaching 50–60% of the total DM cell division and cell expansion processes, which around 120 DAP (Fig. 5.2; Howeler and determine the number of cells per mature leaf Cadavid, 1983; Távora et al., 1995). After the and cell size, respectively. Thus the final leaf area fourth month, more DM is accumulated in is directly affected by the rate and duration of the storage roots than the rest of the plant. At cell division and expansion (Takami et al., 1981; harvest (12 months) DM is present mainly in Lecoeur et al., 1995). A temporal development roots, followed by stems and leaves (Fig. 5.3; of cell division and expansion for adaxial epider- Howeler and Cadavid, 1983). Thus, during the mal cells in cassava has been proposed by Alves growth cycle, the DM distribution to the differ- (1998; Fig. 5.4), in which the transition from ent parts is constant with a high positive linear leaf cell division to cell expansion processes is correlation of the total DM with shoot and root discrete and occurs when leaf area reached 5% DM (Fig. 5.1; Veltkamp, 1985d). of its final size, corresponding to the first folded The period of maximum rates of DM leaf toward the top. Thus when the leaf starts to accumulation depends on genotypes and grow- unfold, almost all cell division stops and rapid ing conditions. Lorenzi (1978) at high latitude cell expansion starts. and Oelsligle (1975) at high altitude reported maximum rates of DM accumulation at 4–6 and 7 months, respectively. Under more tropical Dry matter partitioning and source–sink conditions, where growth is faster, Howeler and relationship Cadavid (1983) found an earlier period of maxi- mum rates, at 3–5 MAP. The distribution of DM During cassava growth the carbohydrates from to the economically useful plant parts is mea- have to be distributed to assure sured by harvest index (HI). In cassava HI repre- good development of the source (active leaves) sents the efficiency of storage root production and provide DM to the sink (storage roots, stem and is usually determined by the ratio of storage and growing leaves). Cassava DM is translocated root weight to the total plant weight. Significant mainly to stems and storage roots, and DM differences in HI have been reported among

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cultivars, indicating that it can be used as a selec- (27° 37′ S), where photoperiods range from 14.8 tion criterion for higher yield potential in cas- to 11.2 h. Concentration of first flowering and sava. HI values of 0.49–0.77 have been reported forking occurred in photoperiods > 13.5 h. This after 10–12 MAP (Lorenzi, 1978; Cavalcanti, result is consistent with Bruijn (1977) and 1985; Pinho et al., 1995; Távora et al., 1995; Cunha and Conceição (1975), who suggested Peressin et al., 1998). Although DM distribution flowering in cassava may be promoted by is constant, its accumulation depends upon increasing day length. photoassimilate availability (source activity) and sink capacity of the storage parts. The number of storage roots and their mean weight are yield components that determine sink capacity. The Photosynthesis significant positive correlation of photosynthetic rate with root yield and total biomass, as well Cassava photosynthesis follows a C3 pathway as the correlations between LAI, interception (Veltkamp, 1985e; Edwards et al., 1990; Angelov of radiation and biomass production (Williams, et al., 1993; Ueno and Agarie, 1997) with 1972; Mahon et al., 1976; El-Sharkawy and maximum photosynthetic rates varying from −2 −1 Cock, 1990; Ramanujam, 1990), indicates that 13 to 24 µmol CO2 m s under greenhouse demand for photoassimilates by roots increases or growth chamber conditions (Mahon et al., the photosynthetic activity. 1977b; Edwards et al., 1990) and from 20 to −2 −1 The balance between ‘source’ and ‘sink’ 35 µmol CO2 m s in the field (El-Sharkawy activity is essential for the plant to reach its and Cock, 1990). It exhibits a high CO2 compen- −1 maximum productivity. Studies have shown sation point, from 49 to 68 µll , typical of C3 that up to 25% reduction in the number of plants (Mahon et al., 1977a; Edwards et al., storage roots did not affect total or root DM and 1990; Angelov et al., 1993). In field-grown the IAF (Cock et al., 1979). On the other hand, cassava, photosynthesis has high optimum Ramanujam and Biradar (1987) observed that temperature (35°C) and wide plateau (25–35°C; reduction of 50–75% in storage roots did affect El-Sharkawy and Cock, 1990) and is not light root growth rate without changing shoot growth saturated up to 1800 µmol PAR m−2 s−1 rate, indicating that shoot growth is independent (El-Sharkawy et al., 1992b) or 2000 µmol PAR of storage root growth. Influence of source size on m−2 s−1 (Angelov et al., 1993). Thus cassava DM production shows that the NAR and storage is adapted to a tropical environment, requiring root growth rate is reduced when the source size high temperature and high solar radiation for is increased from LAI 3.0 to 6.0 (Ghosh et al., optimal leaf development and for expression of 1988). its photosynthetic potential. Both storage root yield and total biomass show positive correlation with photosynthesis rate (El-Sharkawy and Flowering Cock, 1990; Ramanujam, 1990). Morphologically, cassava leaves combine Little is known about flowering in cassava, and some novel characteristics related to high some clones have never been known to flower. productivity and drought tolerance and, conse- Flowering can start 6 weeks after planting quently, to photosynthesis. The lower mesophyll although the precise flowering time depends surface is populated with papillose-type epi- on cultivar and environment. It appears that dermal cells, while the upper surface is fairly cassava flowers best at moderate temperatures smooth, with scattered stomata and . (approximately 24°C). It has been suggested The papillae appear to add about 15% to leaf that forking is related to the onset of flowering, thickness and to lengthen the diffusion path which is promoted by long days in some from the stomatal opening to the bulk air perhaps cultivars. Usually, the apical becomes two- to threefold (Angelov et al., 1993). Cassava reproductive when branching occurs, but the leaves have distinct green bundle-sheath cells, abortion of flowers is very common. with small, thin-walled cells, spatially separ- Keating et al. (1982a) evaluated cassava ated below the palisade cells (different from at 12 different planting dates at a high latitude Kranz-type leaf anatomy). In addition to

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performing C3 photosynthesis, these cells may fermentation, drying or a combination of these function in transport of photosynthates in the processing treatments aid in reducing the HCN leaf. In Kranz anatomy, typical of C4 plants, the concentration to safe levels (O’Hair, 1990). bundle sheaths are surrounded by and all in direct contact with many mesophyll cells (Edwards et al., 1990). Physical Deterioration of Storage Roots

Cyanide Content Cassava roots have the shortest postharvest life of any of the major root crops (Ghosh et al., All cassava organs, except seeds, contain cyano- 1988). Roots are highly perishable and usually genic glucoside (CG). Cultivars with < 100 mg become inedible within 24–72 h after harvest kg−1 fresh weight (FW) are called ‘sweet’ while due to a rapid physiological deterioration pro- cultivars with 100–500 mg kg−1 are ‘bitter’ cess, in which synthesis of simple phenolic com- cassava (Wheatley et al., 1993). The most abun- pounds that polymerize occurs, forming blue, dant CG is linamarin (85%), with lesser amounts brown and black pigments (condensed tannins; of lotaustralin. Total CG concentration depends Wheatley and Chuzel, 1993). It is suggested that on cultivar, environmental condition, cultural polyphenolic compounds in the roots oxidize to practices and plant age (McMahon et al., 1995). quinone-type substances, which is complexed The variation found in some parts of cassava is with small molecules like amino acids to form shown in Table 5.5. coloured pigments that are deposited in the Linamarin, which is synthesized in the leaf vascular bundles (Ghosh et al., 1988). The and transported to the roots, is broken down by accumulation of the coumarin, scopoletin, is the enzyme linamarase, also found in cassava especially rapid, reaching 80 mg kg−1 dry tissues (Wheatley and Chuzel, 1993). When weight (DW) in 24 h (Wheatley and Chuzel, linamarin is hydrolysed, it releases HCN, a vola- 1993). dehydration, especially at sites of tile poison (LD50–60 mg for humans; Cooke and mechanical damage to the roots encourages the Coursey, 1981); but some cyanide can be detoxi- rapid onset of deterioration. The phenolic com- fied by the human body (Oke, 1983). In intact pounds may be released on injury. The changes roots the compartmentalization of linamarase during storage of roots depend upon condition in the and linamarin in cell and duration of storage, physiological state of prevents the formation of free cyanide. Upon the stored material and varietal characteristics processing, the disruption of tissues ensures that (Ghosh et al., 1988). the enzyme comes into contact with its substrate, Polyphenol oxidase (PPO) is an enzyme resulting in rapid production of free cyanide via that oxidizes phenols to quinone. Any process an unstable cyanhydrin intermediary (Wheatley that inhibits PPO, such as heat treatment, cold and Chuzel, 1993). Juice extraction, heating, storage, anaerobic atmosphere and dipping

Table 5.5. Cyanide concentration in different parts of the cassava plant.

Part of plant Total cyanide concentration Source

Root pulp (parenchyma) 3–121 mg 100 g−1 DW Barrios and Bressani (1967) 3–135 mg 100 g−1 DW Wheatley and Chuzel (1993) 1–40 mg 100 g−1 FW Barrios and Bressani (1967) Root peel 6–55 mg 100 g−1 DW Wheatley and Chuzel (1993) 5–77 mg 100 g−1 DW Barrios and Bressani (1967) 17–267 mg 100 g−1 FW Barrios and Bressani (1967) Leaf 1–94 mg 100 g−1 DW Barrios and Bressani (1967) 0.3–29 mg 100 g−1 FW Barrios and Bressani (1967)

DW, dry weight; FW, fresh weight.

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roots in solutions of inhibitors (e.g. ascorbic of cassava under the temperature variations acid, glutathione and KCN) prevents vascular that usually occur where cassava is normally streaking (Ghosh et al., 1988). cultivated indicates that its growth is favourable Secondary deterioration can follow physio- under annual mean temperatures ranging from logical or primary deterioration 5–7 days after 25 to 29°C (Conceição, 1979), but it can harvest. This is due to microbial infection of tolerate from 16 to 38°C (Cock, 1984). Table mechanically damaged tissues and results in 5.6 summarizes the ranges of temperature and the same tissue discoloration with vascular their principal physiological effects on cassava streaks spreading from the infected tissues development. (Wheatley and Chuzel, 1993). At low temperatures (16°C) sprouting of the stem cutting is delayed, and rate of leaf production, total and storage root DW are Environmental Effects on decreased (Cock and Rosas, 1975). Sprouting Cassava Physiology is hastened when the temperature increases up to 30°C but is inhibited with temperatures Cassava is found over a wide range of edaphic >37°C (Keating and Evenson, 1979). As and climatic conditions between 30°N and 30°S temperature decreases, leaf area development latitude, growing in regions from sea level to becomes slower because the maximum size of 2300 m altitude, mostly in areas considered individual leaves is smaller, and fewer leaves marginal for other crops: low-fertility soils, are produced at each apex although leaf life is annual rainfall from < 600 mm in the semiarid increased (Irikura et al., 1979). At a temperature tropics to > 1500 mm in the subhumid and of 15–24°C, the leaves remain on the plant for up humid tropics. Given the wide ecological to 200 days (Irikura et al., 1979), while at higher diversity, cassava is subjected to highly varying temperatures leaf life is 120 days (Splittstoesser temperatures, photoperiods, solar radiation and and Tunya, 1992). rainfall. There is a genotype-by-temperature inter- action for yield ability. Irikura et al. (1979) evalu- ated four cultivars under different temperatures Temperature and found that higher yields were obtained at different temperatures according to the cultivar, Temperature affects sprouting, leaf size, indicating that the effect of natural selection leaf formation, storage root formation and, con- is highly significant on varietal adaptation sequently, general plant growth. The behaviour (Table 5.7).

Table 5.6. Effect of temperature on cassava development.

Air temperature (°C) Physiological effects

< 17 or > 37 Sprouting impaired 28.5–30 Sprouting faster (optimum) < 15 Plant growth inhibited 16–38 Cassava plant can grow 25–29 Optimum for plant growth < 17 Reduction of leaf production rate, total and root DW 20–24 Leaf size and leaf production rate increased; leaf life shortened 28 Faster shedding of leaves; reduction in no. branches 25–30 Highest rates of photosynthesis in greenhouse 30–40 Highest rates of photosynthesis in the field 16–30 rate increases linearly and then declines

Sources: Wholey and Cock (1974); Cock and Rosas (1975); Mahon et al. (1977b); Conceição (1979); Irikura et al. (1979); Keating and Evenson (1979); El-Sharkawy et al. (1992b). DW, dry weight.

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Table 5.7. Fresh root yield (t ha−1) of four contrasting cassava types at 12 months after planting (MAP) under three different temperature regimes.

Temperature (°C)

Variety 20 24 28

M Col 22 9.3 27.7 39.4 M Mex 59 22.8 38.8 30.4 M Col 113 24.2 26.1 23.9 Popayán 28.9 15.7 9.4

Source: Irikura et al. (1979).

The main effect of temperature is on bio- length (Bolhuis, 1966). Long days promote logical production, as DM partitioning does not growth of and decrease storage root change much when cassava is cultivated under development, while short days increase storage different temperatures (Cock and Rosas, 1975). root growth and reduce the shoots, without Higher temperatures are associated with a influencing total DW (Fig. 5.5). The increase greater crop growth rate (CGR) and high photo- in shoot DW under long days is a result of synthetic rate. El-Sharkawy et al. (1992b) significant increases in plant height, leaf area evaluated the potential photosynthesis of three per plant, number of apices per plant, and cultivars from contrasting habits under different number of living leaves per apex (Veltkamp, growing environments and verified that photo- 1985b). This suggests an antagonistic relation- synthetic rate increased with increasing temper- ship between shoot growth and development of ature, reaching its maximum at 30–40°C. In all the storage roots in response to variation in day cultivars photosynthesis was substantially lower length. in leaves that had developed in the cool climate There are varietal differences in sensitivity than in those from the warm climate. The high to long days (Carvalho and Ezeta, 1983). Under sensitivity of photosynthesis to temperature field conditions, Veltkamp (1985b) submitted points to the need for genotypes more tolerant three genotypes to two day lengths (12 and 16 h) to low temperature, which could be used in the during the whole growth period. He observed highland tropics and subtropics. that the percentage decrease in storage root yield under the long day was greatest in M Col 1684 (47%) and least in M Col 22 (13%; Table 5.8). Photoperiod Yield differences resulted mainly from decreased efficiency of storage root production or HI under Day length affects several physiological pro- 16-h days because total DM was greater under cesses in plants. The differences in day length 16-h days. No day-length effect was found for the in the tropical region are very small, varying crop weight at which the starch accumulation in from 10 to 12 h throughout the year. Thus the roots apparently started (AISS values; Table photoperiod may not limit cassava root pro- 5.8). Thus the storage root yield reduction seems duction in this region. On the other hand, the to be more related to a change in the distribution restrictions regarding cassava distribution out- patterns of DM rather than to a delay in storage side the tropical zone can be due to effects of day root initiation. Considering that photoperiod pri- length variation on its physiology. Although marily affects shoots and a secondary response studies about day length effect in cassava are occurs in the roots (Keating et al., 1982b) and scarce, tuberization, photoassimilates partition- that shoot growth has preference over root ing and flowering are reportedly affected. growth (Cock et al., 1979; Tan and Cock, 1979), Experiments in which the day length was long photoperiods may increase the growth artificially changed have shown that the optimal requirements of the shoots, thereby reducing the light period for cassava is around 12 h, with excess carbohydrates available for root growth probable varietal differences in the critical day (Veltkamp, 1985b).

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Fig. 5.5. Effect of day length on cassava dry matter distribution 16 weeks after planting. Vertical lines indicate twice the standard error of total dry weight (DW), stem DW and storage root DW.

Table 5.8. Cassava dry matter (DM) production and distribution 272 days after planting (DAP) under 16 h and natural day length (approximately 12 h).

Total DM DW storage root Harvest Index AISSb Genotype Day length (t ha−1)a (t ha−1) (HI) (t ha−1)

M Col 1684 Naturalc 16.7 9.1 0.54 0.55 16 hd 17.3 4.6 0.27 0.25 M Ptr 26 Natural 14.5 8.1 0.61 0.60 16 h 15.9 4.9 0.42 0.36 M Col 22 Natural 15.5 9.5 0.56 0.65 16 h 19.5 8.3 0.31 0.45

Source: Veltkamp (1985b). aIncludes weight of fallen leaves. bAISS, apparent initial start of starch accumulation; corresponds to crop weight at which starch accumulation apparently starts in roots. cNatural = approximately 12 h. d16 h = 12 h of natural day length + 4 h of artificial light. DW, dry weight.

Solar radiation groundnuts; Mutsaers et al., 1993). Cassava is also intercropped with perennial vegetation The commonest cassava production system is (Ramanujam et al., 1984). Cassava is usually intercropping with other staple crops. In Latin planted after the intercropped species. Even America and Africa, cassava is usually associ- when it is planted at the same time, the associ- ated with an earlier maturing grain crop such as ated crop such as maize is established faster than maize, rice or grain legumes (beans, cowpeas or cassava. Thus in an associated cropping system

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cassava is always subjected to different degrees but under 95–100% shade, leaves abscise within of shading and low light intensity in the early 10 days (Cock et al., 1979). stages of development. Considering that cassava Aresta and Fukai (1984) observed that only is a crop that requires high solar radiation 22% shade decreased both fibrous root elonga- to perform photosynthesis more efficiently (El- tion rate (53%) and storage root growth rate Sharkawy et al., 1992b), it is very important to (36%) without altering shoot growth rate, which know the effect of shade on cassava develop- was significantly decreased (32%) only under ment and production. Ramanujam et al. (1984) 68% shade. Thus under limited photosynthesis evaluated 12 cassava cultivars under the shade caused by low solar radiation, most of the photo- in a coconut garden (85–90% shading). Under synthates are utilized for shoot growth, affecting shading, the root bulking process started about storage root development significantly, showing 3 weeks after that in plants grown without that the shoots are a stronger sink than roots. shading, and the number of storage roots per plant and NAR was reduced under shading. Okoli and Wilson (1986) submitted cassava to Water deficit six shade regimes and observed that all levels of shade delayed storage root bulking and at 20, Cassava is commonly grown in areas receiving 40, 50, 60 and 70% shade reduced cassava yield < 800 mm rainfall year−1 with a dry season of by 43, 56, 59, 69 and 80%, respectively. 4–6 months, where tolerance to water deficit is In relation to shoots, under field conditions, an important attribute. Although it is a drought- shading increases plant height and the leaves tolerant crop, growth and yield are decreased by tend to become adapted to low light conditions by prolonged dry periods. The reduction in storage increasing leaf area per unit weight (Fukai et al., root yield depends on the duration of the water 1984; Okoli and Wilson, 1986; Ramanujam deficit and is determined by the sensitivity of et al., 1984) and shortening leaf life only under a particular growth stage to water stress. The severe shading. Under ideal growing conditions, critical period for water-deficit effect in cassava cassava leaves have a life of up to 125 days is from 1 to 5 MAP – the stages of root initiation (Splittstoesser and Tunya, 1992). Levels of shade and tuberization. Water deficit during at least 2 up to about 75% have very little effect on leaf life, months of this period can reduce storage root

Fig. 5.6. Effect of water deficit during different growth periods on cassava yield.

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yield from 32 to 60% (Connor et al., 1981; Porto 1984; Cock et al., 1985). This response to early et al., 1988). Figure 5.6 shows the root yield stages of soil water depletion has been described reduction caused by water deficit imposed for 2 as isohydric, a behaviour shared by cowpeas, months during successive 2-month periods from maize and several other crops (Tardieu and 1 to 11 MAP. Clearly, they found that the Simonneau, 1998). Leaf area growth is also severer effect corresponded to stress occurring decreased in response to water stress but is from 1 to 5 MAP (i.e. period of rapid leaf growth rapidly reversed following the release from and tuberization) compared with the later stress (Connor et al., 1981; Palta, 1984; period of storage root bulking. El-Sharkawy and Cock, 1987b; Baker et al., 1989). This response limits the development of plant transpirational surface area during water Drought tolerance deficit and keeps sink demand well balanced with plant assimilatory capacity. Plants respond to water deficit at many differ- Leaf conductance to water vapour has been ent levels: morphological, physiological, cellular evaluated as an indicator of the capacity of and metabolic. The responses are dependent different cassava genotypes to prevent water upon the duration and severity of stress, the loss under prolonged drought. Considerable genotype of the stressed plant, the stage of variation has been observed in leaf conductance development, and the organ and cell type in (Porto et al., 1988), and this parameter seems question (Bray, 1994). Multiple responses allow to be useful for pre-selecting sources of germ- the plant to tolerate water stress. Some of these plasm conferring adaptation to prolonged dry responses and the current status of knowledge periods. with regard to cassava are discussed here.

Control of stomatal closure and Abscisic acid accumulation leaf growth Many authors have reported that a substantial A primary response to water stress is stomatal number of drought responses in plants can closure, which decreases photosynthetic CO2 be mimicked by external application of abscisic assimilation and, in turn, growth. Stomata have acid (ABA) to well-watered plants (Davies et al., a high capacity to respond to changes in the 1986; Trewavas and Jones, 1991). This treat- water status of the plant and atmosphere. They ment promotes characteristic developmental close as the leaf water potential decreases and changes that can help the plant cope with water when the vapour pressure deficit between the deficit, including decrease of stomatal con- leaf and the air increases (generally due to a ductance (MacRobbie, 1991; Trejo et al., 1993), decrease in relative humidity). As stomata are restriction of shoot growth (Creelman et al., the route by which CO2 enters the leaf, stress- 1990) and leaf area expansion (Van Volken- induced decreases in stomatal aperture can burgh and Davies, 1983; Lecoeur et al., 1995) limit the rate of CO2 diffusion into the leaf and, and stimulation of root extension (Sharp et al., therefore, the rate of photosynthesis. 1993, 1994; Griffiths et al., 1997). All these When water is available, cassava maintains effects of ABA application, together with the a high stomatal conductance and can keep observation that environmental stress stimu- internal CO2 concentration high; but when lates ABA biosynthesis and ABA release from water becomes limiting, the plant closes stomata sites of synthesis to action sites, suggest a role in response to even small decreases in soil water for ABA as a stress hormone in plants. More- potential (El-Sharkawy and Cock, 1984). The over, studies have indicated that for certain rapid closure of the stomata and the resulting plant responses, sensitivity to water deficit is decline in transpiration lessens the decrease correlated with changes in ABA concentrations in leaf water potential and soil water depletion, (Trejo et al., 1995; Borel et al., 1997) and geno- thereby protecting leaf tissues from desiccation typic responsiveness to ABA (Blum and Sinmena, (Ike, 1982; Palta, 1984; El-Sharkawy and Cock, 1995; Cellier et al., 1998). Information about

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ABA accumulation in cassava has not been Osmotic adjustment reported in the literature. Alves and Setter (2000) published the One means of increasing drought tolerance is first report concerning ABA in cassava. They by accumulation of osmotically active solutes, cultivated five cassava genotypes in pots in a so that turgor and turgor-dependent processes greenhouse and evaluated the temporal patterns may be maintained during episodes of drydown. of ABA accumulation in mature leaves and in Osmotic adjustment (OA), defined as the differ- immature expanding leaves, during water deficit ence in osmotic potential between control and and after release from stress to determine the stressed plants, allows cell enlargement and extent to which the stress and recovery response plant growth at high water stress and allows of leaf area growth is associated with the tempo- stomata to remain partially open and CO2 assim- ral pattern of ABA accumulation. At 3 and 6 ilation to continue at low water potentials that days after withholding irrigation, all genotypes are otherwise inhibitory (Pugnaire et al., 1994). accumulated large amounts of ABA in both In cassava OA has not been thoroughly expanding and mature leaves, but these high examined. Although leaf water potential ABA levels were almost completely reversed in remains relatively unchanged during water respect of control levels after 1 day of rewatering deficit episodes (Connor and Palta, 1981; Cock (Fig. 5.7). Correspondingly, young leaves halted et al., 1985; El-Sharkawy et al., 1992a), a result leaf expansion growth and transpiration rate suggestive of little or no OA, such observations decreased. Young leaves accumulated more do not rule it out. Furthermore, these studies ABA than mature leaves both in control and involved mature leaves, not young organs, stressed treatments (Fig. 5.7). This rapid return which might especially benefit from accumula- to control ABA levels corresponded with a rapid tion of osmolytes. Alves (1998) evaluated the recovery of leaf area growth rates. This rapid role of OA in cassava by measuring the osmotic reduction in leaf area growth and stomatal component of leaf water potential in mature, closure may be due to cassava’s ability to expanding and folded leaves. The largest synthesize rapidly and accumulate ABA at an increase in solutes caused by water deficit early phase of a water-deficit episode. occurred in the youngest tissue (folded leaves)

Fig. 5.7. Abscisic acid (ABA) concentration in expanding and mature cassava leaves in control after 3 and 6 days of water deficit, followed by 1 and 3 days of rewatering; average of five genotypes with three replicates; bars represent SEM (n = 15); data from Alves and Setter (2000).

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and the extent of OA increased progressively Borel, C., Simonneau, T., This, D. and Tardieu, F. from mature to folded leaves. As young tissues (1997) Stomatal conductance and ABA con- () are involved in regrowth and centration in the xylem of barley lines of recovery after drought, further research is contrasting genetic origins. Australian Journal of 24, 607–615. needed to give a fuller picture of cassava’s Bray, E.A. (1994) Alterations in gene expression in responses to water deficit. response to water deficit. In: Basra, A.S. (ed.) Stress-Induced Gene Expression in Plants. Hard- Academic Publishers, Chur, Switzerland, References pp. 1–23. Bruijn, G.H. (1977) Influence of daylength on the Alves, A.A.C. (1998) Physiological and developmental flowering of cassava. Tropical Root Crops changes in cassava (Manihot esculenta Crantz) Newsletter 10, 1–3. under water deficit. PhD thesis, Cornell Univer- Cardoso, C.E.L. and Souza, J. da S. (1999) Aspectos sity, Ithaca, New York, 160 pp. Agro-Econômicos da Cultura da Mandioca: Alves, A.A.C. and Setter, T.L. (2000) Response of Potencialidades e Limitações. Documentos cassava to water deficit: leaf area growth and CNPMF no. 86, EMBRAPA/CNPMF, Cruz das abscisic acid. Crop Science 40, 131–137. Almas, BA, Brazil, 66pp. Angelov, M.N., Sun, J., Byrd, G.T., Brown, R.H. and Carvalho, P.C.L. de and Ezeta, F.N. (1983) Efeito do Black, C.C. (1993) Novel characteristics of fotoperíodo sobre a ‘tuberização’ da mandioca. cassava, Manihot esculenta Crantz, a reputed Revista Brasileira de Mandioca 2, 51–54. C3–C4 intermediate photosynthesis species. Cavalcanti, J. (1985) Desenvolvimento das raízes Photosynthesis Research 38, 61–72. tuberosas em mandioca (Manihot esculenta Aresta, R.B. and Fukai, S. (1984) Effects of solar radia- Crantz). MSc thesis, Universidade Federal do tion on growth of cassava (Manihot esculenta Ceará, Fortaleza, Brazil, 66 pp. Crantz). II. Fibrous root length. Field Crops Cellier, F., Conéjéro, G., Breitler, J.C. and Casse, F. Research 9, 361–371. (1998) Molecular and physiological responses to Baker, G.R., Fukai, S. and Wilson, G.L. (1989) The water deficit in drought-tolerant and drought- response of cassava to water deficits at various sensitive lines of sunflower. Accumulation of stages of growth in the subtropics. Australian dehydrin transcripts correlates with tolerance. Journal of Agricultural Research 40, 517–528. Plant Physiology 116, 319–328. Barrios, E.A. and Bressani, R. (1967) Composición Cerqueira, Y.M. (1989) Efeito da deficiência de água química de la raíz y de la hoja de algunas na anatomia foliar de cultivares de mandioca variedades de yuca Manihot. Turrialba 17, (Manihot esculenta Crantz). MSc thesis, Univers- 314–320. idade Federal da Bahia, Cruz das Almas, Brazil. Beeching, J.R., Marmey, P., Gavalda, M.C., Noirot, M., CIAT (1984) Morphology of the Cassava Plant; Study Haysom, H.R., Hughes, M.A. and Charrier, A. Guide. Centro Internacional de Agricultura (1993) An assessment of genetic diversity within Tropical, Cali, Colombia. a collection of cassava (Manihot esculenta Crantz) Cock, J.H. (1976) Characteristics of high yielding germplasm using molecular markers. Annals of cassava varieties. Experimental Agriculture 12, Botany 72, 515–520. 135–143. Blum, A. and Sinmena, B. (1995) Isolation and charac- Cock, J.H. (1984) Cassava. In: Goldsworthy, P.R. terization of variant wheat cultivars for ABA and Fisher, N.M. (eds) The Physiology of Tropical sensitivity.Plant,CellandEnvironment18,77–83. Field Crops. John Wiley & Sons, Chichester, Boerboom, B.W.J. (1978) A model of dry matter distri- pp. 529–549. bution in cassava (Manihot esculenta Crantz). Cock, J.H. and Rosas, S. (1975) Ecophysiology of Netherlands Journal of Agricultural Science 26, cassava. In: (eds) Symposium on Ecophysiology 267–277. of Tropical Crops. Communications Division of Bolhuis, G.G. (1966) Influence of length of the illumi- CEPLAC, Ilhéus, Bahia, Brazil, pp. 1–14. nation period on root formation in cassava Cock,J.H.,Franklin,D.,Sandoval,G.andJuri,P.(1979) (Manihot utilissima Pohl). Netherlands Journal of The ideal cassava plant for maximum yield. Crop Agricultural Science 14, 251–254. Science 19, 271–279. Bonierbale, M., Guevara, C., Dixon, A.G.O., Ng, N.Q., Cock, J.H., Porto, M.C.M. and El-Sharkawy, M.A. Asiedu, R. and Ng, S.Y.C. (1997) Cassava. In: (1985) Water use efficiency of cassava. III. Fuccillo, D., Sears, L. and Stapleton, P. (eds) Influence of air humidity and water stress on gas Biodiversity in Trust. Cambridge University Press, exchange of field grown cassava. Crop Science 25, Cambridge, pp. 1–20. 265–272.

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